Sit-up's will burn calories, believe me, you're working a very broad range of muscles in your body...but you'd be better off going for a light walk, 15-20 minutes.
Abs are not super-muscles, and shouldn't be worked out two days in a row. They need rest to repair themselves and get bigger.
If you're trying to see your abs, no amount of sit-up's will help until you have a body-fat percentage low enough to reveal them.

Sit-up's will burn calories, believe me, you're working a very broad range of muscles in your body...but you'd be better off going for a light walk, 15-20 minutes.
Abs are not super-muscles, and shouldn't be worked out two days in a row. They need rest to repair themselves and get bigger.
If you're trying to see your abs, no amount of sit-up's will help until you have a body-fat percentage low enough to reveal them.

Abs CAN be worked out daily, one of the few muscles that can, but situps alone are useless, you should be doing ball crunches, leg lifts, crunches, and a number of different exercises.

To burn calories run, sit ups wont accomplish what you need. And like Game 7 is saying you need a very low body fat % to see a 6 pack.

No. Abs aren't some magical group of muscles that can withstand anything. If you work them hard enough, they get sore. If you're sore, you have to let the muscles recover. If they aren't sore, then you probably CAN work on them day after day, but that's only because you're not getting a really hard workout that challenges the repaired and, now, stronger muscles.

But abs aren't some unique muscles. They need rest like every other muscle.

Anyway, some great ways to burn more calories include running and jump rope. Those are probably the two best. Running, however, is hard on the knees. Jumping rope can be hard on the lower half, but, from my experience, not nearly as hard on the knees as running (especially if you are running on concrete with shoes).